1. Field of the Invention
This invention relates to a battery charger and battery charging method, and more particularly, is suitably applied to a battery charger having a voltage regulating circuit and a current regulating circuit.
2. Description of the Related Art
Secondary batteries have been used as a power source in portable electronic apparatus such as a portable telephone, a camera integrated VTR, and so on. When these portable electronic apparatus are used, the secondary batteries must be charged prior to their use.
When the secondary battery is charged, a secondary battery is likely to be damaged if applied with a charging voltage and a charging current above its rated values. Therefore, a battery charger generally has a voltage regulating circuit and a current regulating circuit for preventing a secondary battery from being applied with a charging voltage and a charging current above its rated values.
Here, such a battery charger is shown in FIG. 1. As shown in FIG. 1, a battery charger 1 has a current regulating circuit 2 and a voltage regulating circuit 3 which are connected in series, wherein an input voltage and an input current from an alternating-current transformer (AC adapter) 4 connected thereto as a direct current power source are used to supply a secondary battery BT with a charging current at or below a rated value through the current regulating circuit 2 and to supply the secondary battery BT with a charging voltage at or below a rated value through the voltage regulating circuit 3.
The battery charger 1 configured as described above, however, has a problem in that while the current regulating circuit 2 and the voltage regulating circuit 3 can prevent the secondary battery BT from damages, the current regulating circuit 2 and the voltage regulating circuit 3 themselves cannot be protected from being damaged. Generally, as a direct current power source to be supplied to the battery charger 1, a direct current power source having an output voltage and an output current adapted to the battery charger 1 is used. However, if a user uses a direct current power source having an output voltage and an output current not adapted to the battery charger 1, the current regulating circuit 2 and the voltage regulating circuit 3 are likely to receive an input current and an input voltage above their rated values, thereby causing a problem in that heat above a rated value occurs in the current regulating circuit 2 and the voltage regulating circuit 3 to cause damages in the current regulating circuit 2 and the voltage regulating circuit 3.